1. Field of the Invention
The present invention relates to an optical disk apparatus which records and reproduces information by irradiating an optical beam to a rotating optical disk, and more particularly, to a tracking servo technique for tracing exactly the optical beam on a track on the optical disk.
2. Description of the Related Art
In an optical disk recording/playback apparatus, a tracking servo technique, which generates a tracking signal and controls the position of an optical pickup in the disk radius direction based on the tracking signal, is indispensable for tracing an optical beam on a track having an array of pits to indicate information.
In a read-only optical disk such as a DVD-ROM, a pit to indicate information is recorded as a hole for example by press work. In a recordable optical disk such as a DVD-RAM, a group is formed spirally and a track is formed by a group as a concave and a land as a convex. A beam is irradiated to the track, and a pit to indicate information is recorded as a phase-changed portion on the disk surface.
A method of generating a tracking error signal is available in differential phase detection (hereinafter referred to as DPD) and push pull (hereinafter referred to as PP). In a play-only DVD optical disk apparatus, a tracking error signal is generally generated by a DPD method. In a recording/playback CD and DVD recording optical disk apparatus, a tracking error signal is mostly generated by a PP method.
The optimum depth of a pit is different according to the tracking error signal generating methods. The optimum pit depth is also different according to the disk types, CD or DVD. Further, if the pit depth varies as a result of molding an uneven disk, the diffraction light intensity varies degrading the tracking signal accuracy.
In the PP method, for example, if the wavelength of an optical beam irradiated to a disk is assumed to be λ, the amplitude of a tracking error signal will be theoretically 0 when the pit depth is λ/4. Therefore, when the pit depth is near λ/4, the tracking servo becomes difficult in the PP method.